Corporations are composed of and supported by people- I see nothing wrong with defining organizations of any kind as people under the Constitution.

Are corporations and trade unions and pension funds too big? If they are limited in their efforts to spread their opinions, where should the cut-off be placed? Individual? Family? Local society? Organizations with 50 or more employees? Those with 50,000 employees?
The idea of selective limitations, individual(s) vs. their organizations, is ridiculous.